To date this project has made a number of contributions to our knowledge of metastatic breast cancer. First, it demonstrated that the dissemination of tumor cells has a significant inherited component, contrary to the conventional theory that metastasis resulted from a purely somatic evolutionary process. Second, the first metastasis susceptibility gene, Sipa1, was identified in mouse models and was subsequently demonstrated to be associated with progression in human populations as well. Third, a number of additional genes were discovered that also contribute to human metastatic susceptibility. Genetic analysis of these genes has revealed the presence of at least two different mechanisms for metastatic spread. Work over the past year has continued to build on this basis. Utilizing the genetic metastasis susceptibility mapping strategy pioneered in our laboratory we have contributed to the analysis of environmental factors in tumor progression, focusing on dietary fat intake. These studies have demonstrated, consistent with previous suggestive human population studies, that high-fat western diets promote metastatic dissemination. These effects are mediated by both tumor autonomous and non-autonomous factors. In addition, we have contributed to additional studies examining the expression patterns and signatures associated with metastatic spread and response to chemotherapy. Furthermore, in collaboration with a number of other laboratories at NCI, we assisted in the developing a novel strategy for imaging conversion of disseminated single tumor cells to proliferating lesions. This new technology accurately replicates response to chemotherapy and therefore may provide an important tool for the ex vivo modeling of metastatic disease. Primary efforts of our laboratory continue to be focused on gene discovery and characterization. Most recently this resulted in the publication of a structure-function analysis of one of the most significant metastasis susceptibility genes, Brd4. Unexpectedly, a poorly defined carboxy-terminal domain was found to mediate epithelial-to-mesenchymal transition and acquisition of a variety of stem cell traits. This data suggests that Brd4 may play a pivotal role in maintenance of a differentiated state in tumor cells. Further efforts are currently in progress to further characterize this domain. Current efforts include five additional manuscripts that are in preparation. These efforts include the identification of two novel metastasis susceptibility genes, a metastasis associated microRNA and a systems biology analysis of the transcriptional networks underlying metastatic disease. Details of these efforts will be presented in subsequent annual reports.